The present invention relates to an apparatus and a method for coating zinc on steel sheets for use on automobiles and electronic apparatuses. More specifically, the present invention relates to an apparatus and a method for coating zinc on steel sheets by using zinc powders.
Zinc performs a sacrificing action for steel to extend the life expectancy of steel, and therefore, conventionally zinc has been used in zinc-coating a steel sheet. There are many kinds of zinc coating methods such as hot dip galvanization, electroplating, and zinc powder-using zinc coating. The zinc coated steel sheets are mostly used as automobile body sheets and outer and inner sheets of electronic apparatuses. They are manufactured by electroplating or hot dip galvanization. The reason is as follows. That is, when a steel strip is continuously coated, not only the product quality but also the productivity and the workability have to be considered. In this respect, electroplating and hot dip galvanization are advantageous.
Electroplating is carried out in the following manner. That is, cold rolled steel sheets are made to undergo a batch annealing or a continuous annealing so as to improve mechanical properties. Then, electroplating is carried out within an electrolyte containing zinc ions, thereby obtaining a zinc deposition layer of the target thickness. In this method, the mechanical properties which have been obtained through heat treatments are not degraded during the plating process. Further, the coated amount (deposition thickness) is varied in accordance with the applied electric power, and therefore, the coated amount can be accurately controlled.
However, it has the following disadvantages. That is, as the coated amount is increased, much more electric power is required. Consequently, the productivity is aggravated in the case of a thick plating. Therefore, electroplating is adopted in the case where the coated amount is 40 g/m.sup.2 or less for one face of the steel sheets. Meanwhile, the plating speed is limited by the current density limit, and therefore, if the productivity is to be improved, the zinc coating chamber has to be long. This becomes a facility burden. Because of such limiting factors, electroplating should be preferably carried out at a strip velocity of about 200 m/min and at a coated amount of 40 g/m.sup.2 or less for one face of the steel sheet.
Meanwhile, hot dip galvanization is carried out in the same facility as that of the annealing. Therefore the manufacturing cost becomes lower, and a thick zinc coating is possible. However, it has the following disadvantages. That is, a sink roll and a guide roll which are immersed in a hot dipping pot are corroded, and therefore, they have to be replaced periodically. Further, as the line speed becomes fast, the resistivity of the melted zinc is increased. Therefore, the sink roll cannot move synchronously with the steel sheet to produce slips, and therefore, the surface of the steel sheet may be scratched so much as to lead to a product defect. Further, as the line speed is increased, or as the coated thickness is decreased, splashes are increased during an air wiping, with the result that the generation of dross is increased. Besides, if the zinc adhered on the surface is to be solidified, some cooling period is required, and therefore, the velocity of the steel sheet is limited to about 200 m/min. Further, the adjustment of the coated thickness is difficult, and therefore, the manufacturing becomes difficult if the coated amount per face is less than 40 g/m.sup.2.
A method for coating zinc by using zinc powders is disclosed in Japanese Patent Application Laid-open No. Hei-5-311388.
This method is illustrated in FIG. 1. As shown in this drawing, an object to be coated (steel sheet) 1 is heated to above the melting point of the powder metal, and zinc powders loaded on a gas are spouted by means of a spouting nozzle 6 onto the object 1 within a zinc coating chamber 3 containing a reducing atmosphere. Thus the zinc powders are melt-adhered on the steel sheet 1, thereby zinc-coating the steel sheet.
In FIG. 1, reference codes 4 and 5 indicate sealing devices.
In this method, a reducing atmosphere is used, and therefore, a flux does not have to be used. Further, compared with the hot dip galvanization, the air wiping and the management of the melt composition are not required, and the dross generation does not occur. However, in the case of Japanese Patent Application Laid-open No. Hei-5-311388, the zinc powders from the powder storage chamber are not screened but directly pouted into-the zinc coating chamber. Therefore, large articles and coarse secondary particles can adhere on the steel sheet, with the result that the coated layer becomes irregular.
Meanwhile, another zinc coating method is disclosed in which an object to be coated is heated to 775.degree. F. (413.degree. C.) -820.degree. F. (438.degree. C.), and zinc powders or a zinc melt is spouted, so that zinc would be coated on steel sheets (CA 866153 (7113).
In this method, however, a flux is spouted together with the zinc powders to prevent the oxidation of the steel sheet. Further, in order to improve the adherence of zinc, electrostatic charges of opposite polarities are provided on the zinc powders and the object to be coated.
In this method, owing to the mechanical spouting force and the electrostatic attractions, a large coated amount can be easily obtained. Further, it can be applied to a complicated steel structure, but when it is applied to a continuous zinc coating of a steel strip, the following problems occur. (1) The high voltage electrostatic charges of opposite polarities are dangerous to workers. (2) There are necessarily loosely adhered zinc particles after the zinc coating, and these particles adhere on various rolls to cause defects called "dent". (3) Zinc powders are released into the external air to aggravate the working environment.
Further, there are other disclosures in which electrostatic charges are utilized in coating zinc powders (U.S. Pat. Nos. 5,384,165 and 5,551,981).
In these methods, zinc powders are made to adhere on steel sheets by utilizing electrostatic charges, and then, the steel sheets are heated to convert the adhered zinc powders to a coated layer.
The apparatus for these methods is illustrated in FIG. 2. As shown in this drawing, the apparatus includes a fluidized bed 18 of zinc powders, and a cooling device 24 and a heating device 21 disposed above the fluidized bed 18. A steel sheet immersed in fluidized bed 18 is made to shift its advancing direction upward to be heated by the heating device 21, so that the zinc powders would be melted. The melted zinc powders are reflowed, and then are cooled.
In FIG. 2, reference code 10 indicates a housing, 16 indicates a strip bending roll, 17 indicates a fall space, 17A indicates a plate as a part of an electrostatic charge circuit, 17B indicates a controller, 20 indicates a top deflector roll.
In these U.S. patents, steel sheets can be coated without much modification to the existing melting facility, but have the following disadvantages.
(1) When the electrostatically charged metal powders contact with the steel sheet, the surface charges are transferred to the steel sheet so as to be grounded and to disappear. Therefore, the attractive force of the electrostatic charges which is to act as the adhering force between the steel sheet and the zinc powder is dissipated. Therefore, the zinc powders depart from the surface of the steel sheet, and therefore, there is a limit in the increase of the coated thickness. PA1 (2) The roller is immersed in the fluidized bed of zinc powders, and therefore, when the steel sheet moves, the zinc powders intrude in between the roller and the steel sheet, so that the zinc powders may adhere on the roller. Particularly, zinc powders speedily undergo sintering reactions above 250.degree. C. Therefore, the zinc powders which have intruded in between the steel sheet and the roller undergo a sintering reaction owing to the latent heat of the steel sheet. As a result, coarse particles may be formed, and the dent phenomenon becomes more serious. PA1 (3) The fine zinc particles of 5-15 .mu.m which are used in the above patents are not well fluidized, but agglomerations occur. Therefore, the zinc particles of the fluidized bed are liable to be irregular, and therefore, if the steel sheet is put into the fluidized bed, a uniform coated layer cannot be obtained. PA1 a zinc coating chamber for forming a fluidized bed of zinc powders, for passing a heated steel sheet through the fluidized bed of the zinc powders, and for making the zinc powders melt-adhere on the steel sheet during the passing of the steel sheet through the fluidized bed; PA1 a fluidized bed forming chamber for forming a fluidized bed of the zinc powders by making the zinc powders suspended by spouting a gas; PA1 a cyclone for separating the zinc powders from the gas after recovery of them from the zinc coating chamber, to discharge the gas, and to return the separated zinc powders to the fluidized bed forming chamber; PA1 a deflector for shifting the advancing direction of the steel sheet after its admittance into the zinc coating chamber; PA1 a tension roll for shifting the advancing direction of a zinc coated steel sheet; PA1 the zinc coating chamber including: a powder inlet tube connected from a side wall of the zinc coating chamber to the fluidized bed to inject the zinc powders into the zinc coating chamber; a gas inlet tube for forming a turbulent flow of the zinc powders and for preventing a leakage of the zinc powders; and a recovering tube for reusing uncoated zinc powders; PA1 the gas inlet tube being disposed above the powder inlet tube, and the recovering tube being disposed below the powder inlet tube; PA1 the recovering tube being connected between the zinc coating chamber and the cyclone, and a suction pump being connected to the cyclone; PA1 a separating plate provided within the zinc coating chamber, for making the uncoated zinc powders smoothly flow to the recovering tube, and for preventing the zinc powders from flowing into the zinc coating chamber after passing through the recovering tube; and PA1 a stabilizing roll disposed below the separating plate. PA1 a zinc coating chamber for making zinc powders melt-adhere on a heated steel sheet to form a coated layer; PA1 a fluidized bed forming chamber for forming a fluidized bed of the zinc powders by making the zinc powders suspended by spouting a gas; PA1 a cyclone for separating the zinc powders from the gas after recovery of them from the zinc coating chamber, to discharge the gas, and to return the separated zinc powders to the fluidized bed forming chamber; PA1 a deflector roll for shifting the advancing direction of the steel sheet after its admittance into the zinc coating chamber; PA1 a tension roll for shifting the advancing direction of a zinc coated steel sheet; PA1 the zinc coating chamber including a powder spouting tube connected from a side wall of the zinc coating chamber to the fluidized bed to spout the zinc powders into the zinc coating chamber; PA1 the zinc coating chamber further including a recovering tube connected to the cyclone, for recovering uncoated zinc powders; and PA1 one or more of electrodes provided in the zinc coating chamber, for electrostatically charging the zinc powders, the electrodes being connected to a high voltage generating device. PA1 supplying fluidized zinc powders of a fluidized bed to the zinc coating chamber, and injecting an inert gas or a reducing gas into the zinc coating chamber through a side wall of the zinc coating chamber to form a fluidized bed within the zinc coating chamber; PA1 making a heated steel sheet (heated to 420-730.degree. C.) through the fluidized bed within the zinc coating chamber to melt-attach the zinc powders on the steel sheet so as to form a coated layer; PA1 reheating the zinc powder-adhered steel sheet at a temperature of 420-650.degree. C. for 1-20 seconds to make residual uncoated zinc powders melt-adhered on the surface of the steel sheet so as to form a coated layer; and PA1 discharging residual uncoated zinc powders from a bottom portion of the zinc coating chamber together with a gas by a cyclone, to separate the zinc powders from the gas so as to discharge the gas and so as to return the separated zinc powders to a fluidized bed forming chamber. PA1 receiving zinc powders from a powder supply device, and fluidizing the zinc powders within a fluidized bed forming chamber by a help of a gas blown through a lower portion; PA1 injecting the fluidized zinc powders from the fluidized bed forming chamber to a zinc coating chamber by means of an injecting device to form a fluidized bed within the zinc coating chamber; PA1 charging the zinc powders of the fluidized bed positively or negatively; PA1 heating a steel sheet to 420-730.degree. C. and grounding the steel sheet, and making the charged steel sheet pass through the fluidized bed to make the zinc powders melt-adhere on the steel sheet; PA1 sending residual zinc powders of a bottom portion of the zinc coating chamber to a cyclone together with a gas to separate the zinc powders from the gas, so as to discharge the gas, and so as to return the separated zinc powders to a powder supply device.
Further, if a reflowing is carried out after the adherence of the zinc powders, a volume contraction occurs as in the case of the powder metallurgy. Therefore, the coated layer may look as if the steel sheet has cracked. Further, if the reflowing is imperfectly carried out, the residual zinc powders of the surface will adhere on the roller so as to form a dent defect.